An electric power steering (EPS) system provides an electric motor coupled to the steering gear and responsive to driver torque input through the steering column to provide power assistance. An electronic control unit with an inverter controls the motor. To fulfil the adequate power assistance a target motor torque is calculated responsive to the driver torque input. The inverter feeds the motor with phase currents for torque generation with either a subharmonic method, space vector modulation or a hysteresis current controller. As result a harmonic and nonharmonic disturbance spectrum in the phase currents and/or the drive torque is generated. This spectrum is undesirable because it leads to acoustic anomalies and torque anomalies in the overall steering system. The described phenomenon is known as torque ripple and it is caused by deviations of the induced electric fields from a sinusoidal shape or not ideal behaviour of current measurement offset or gain errors. Even when the torque ripple is made sufficiently small, that it would not be considered a problem in most applications, the motor output in many EPS systems is provided to a steering component closely coupled to the operator handwheel; and even small torque ripple and housing vibrations can be felt and/or are audible by the vehicle operator in high torque assist steering modes such as low speed parking manoeuvres.
In some EPS the drive torque of the motor will be controlled in a feedback loop to the demand motor torque. The control loop uses a target motor torque processing the control operation. The target motor torque is often also called a reference motor torque. Demand, target and reference will be used as a synonym in this document. Nevertheless the demand motor torque and the target motor torque are not static values. The demand motor torque and the target motor torque are dynamic values depending on the driver torque input and perhaps other input values.
Therefore, in order to suppress vibration and noise generated by the motor or other components in the EPS, various countermeasures are conventionally taken in accordance with the cause of the vibration and noise. For example, the European patent EP 2 000 389 B1 discloses an electric power steering apparatus including a steering force assist device, which applies to the steering system an assist force for assisting a steering operation of a steering wheel in a vehicle steering system, and a control means for controlling actuation of the steering force assist device. The control means includes attenuating means, which attenuate a specific frequency component in accordance with the rotational angular velocity of the motor in order to reduce torque ripples of order components and suppress generation of vibration and noise. However, this procedure is complex and affects the dynamic behaviour of the motor.
Thus a need exists for an electric power steering apparatus that easily and accurately reduces torque ripples and effectively suppresses generation of vibration and noise, with less limiting the dynamic behaviour of the motor in relation to the prior solutions.